This disclosure relates generally to the generation of electricity at facilities storing liquid hydrogen, and especially relates to the use of boiled off waste hydrogen gas as a fuel to generate electricity.
Hydrogen has many uses in industrial and energy applications. In many applications, it is desired to store hydrogen in its liquid form to minimize the amount of space required for storage tanks. Since liquid hydrogen is a cryogenic fluid, having a boiling point below 259° C., even when stored in vacuum-jacketed tanks, the hydrogen will evaporate, or “boil-off” at a fairly constant rate.
Standard liquid hydrogen tanks are fitted with pressure-relief valves which maintain gas pressure from the boil-off below a predetermined safe working level of the tank. Once this pressure is reached, the valve opens and vents the hydrogen gas to the atmosphere. By allowing the hydrogen gas to vent to the atmosphere, a considerable amount of energy, both chemical and kinetic, are lost.
It is estimated that a well maintained liquid hydrogen storage tank losses approximately 1% of its liquid hydrogen capacity each day due to boil-off hydrogen gas. For a 15,000 gallon tank, this translates to 17,000 standard cubic feet (scf) of hydrogen gas per day which is the energy equivalent to nearly 45 gallons of gasoline.
Several concepts have been proposed to recover the boil-off gas by recirculation of the gas into the liquification processing plant to convert the gas back into a liquid phase. While still others have suggested using the boil-off gas as a fuel for use with hydrogen powered vehicles. Both of these ideas have some merit, but also suffer from substantial drawbacks due to the limited number of locations where these concepts would be applied. Reliquification would only be feasible for gas recovered at the processing plant. Often, however, the liquid hydrogen is stored remotely from the processing plant, such as at distant industrial gas depot closer to the end-users. In these cases it would be impractical at best to return recovered gas to the liquification facility.
Using recovered hydrogen gas to power vehicles would seem like an excellent use for a product that would otherwise be disposed. However, at the present time, and for the foreseeable future, there are only a small number of these vehicles available. This combined with the fact that industrial gas depots tend to be located in industrial areas making it inconvenient for the owners of these vehicles to refill their vehicles. Furthermore, the hydrogen gas would need additional compression storage equipment to make it usable as fast fill fueling station for vehicles. These additional equipment requirements would add to the cost and further reduce the benefits of using the waste gas.
What is needed in the art is a system for recapturing some of the energy lost during the storage of liquid hydrogen and transforming the energy into a useful form for use by the storage facility.